82
(1) The water volume of the lake with a given
amount of dissolved species is reduced. This occurs
when evaporation from the lake surface exceeds river
inflow and precipitation onto the lake surface (evaporative concentration, ephemerallake). As a result, the
lake shrinks and may even dry out and form an inland sabkha (see below). This process takes a relatively short time in which dissolved matter of further
inflowing water plays a minor role. The amount of
salt minerals which can be precipitated during one
cycle of evaporation therefore mainly depends on the
pre-existing volume and salt concentration of the
lake.
(2) The lake water volume remains more or less constant (perenniallake). The salt concentration slowly
increases due continuous river influx. Large drainage
areas with high chemical denudation rates (cf. Sect.
9.2) add more dissolved matter to the lake than small
ones in which weathering proceeds slowly.
Principally, mineral precipitation in a lake can commence when the concentrations of the dissolved species with respect to a specific mineral have reached
saturation (thermodynamic equilibrium). This process also depends on the concentrations of other dissolved species as well as on the temperature and pR
of the system. These relationships are special topics
in geochemistry.
The most important types of salt deposits and their
possible temporal and vertical successions in perennial and ephemeral lakes are summarized in Fig.
2.31. Since almost alllakes receive solid loads from
their drainage areas, reactions of so lids with lake water should be considered first. In fact, the incoming
clay minerals, degraded by weathering in the drainage areas, react with lake water and take up cations,
hydrogen carbonate and silica. This so-called
"re gradation" of clay minerals (or reverse weathering) consumes a significant amount of the incoming
dissolved matter.
It may lead to the formation of illite, montmorillonite, and
smectite at the expense of kaolinite. In addition, small
amounts of new clay minerals can form in the lake already
at very low salt concentrations. These processes have been
largely neglected in previous studies on lake evaporites.
The first mineral which is precipitated either chemically or biochemically is normal low-Mg calcite, regardless of the rock types present in the drainage areas. This is so because all river waters and shallow
groundwaters are rich in Ca and RC0 3 and mostly
show Ca>Mg and RC0 3 >Ca+Mg (cf. Sect. 9.2, Figs.
9.6 and 9.8). Increasing salt concentration, e1evated
temperature, and loss of CO 2 favor calcite precipitation.
The next steps in lake evolution are related to the
more specific chemistry of the inflowing water. Two
Chapter 2 Continental Sediments
or three different lines of deve10pment can be dis tinguished (Fig. 2.31):
- The inflowing water is of the common, earth alkalihydrogen carbonate type which is characteristic for
many rivers draining carbonate-bearing sedimentary
rocks and plutonic rocks in highlands. As a result of
calcite precipitation, Mg is re1atively enriched in the
solution. Prograding evaporation leads to the precipitation of further earth alkali carbonates (carbonate
lakes), but now Mg is partially used to transform calcite into other carbonate minerals, e.g. dolomite (see
below). Later, gypsum, halite, Mg salts and other
evaporites form. The ultimate brine is enriched in Na,
K, Mg, Cl and possibly S04 if sulfate reduction is
limited or absent. This brine is poor in RC03 and
Si0 2 and has a relatively low pR.
- A second, less common line of salt deposits starts
with waters comparatively rich in sulfate andlor chloride as well as calcium, but poor in RC03
(RCO)
evaporites in the drainage areas of the lakes (cf. Rio
Grande in North America, cf. Fig. 9.5). Calcite precipitation is followed by gypsum and NazS0 4 minerals inc1uding glauberite (sulfate lakes). The final
stage is again characterized by halite and other highly
soluble mineral:.;.
- The prerequisite for a third series of salts is lake
water rich in hydrogen carbonate and alkali ions.
Such waters often come from drainage areas dominated by volcanic and plutonic rocks inc1uding regions where Na is enriched by cation exchange in the
subsurface. After a sodalake stage with the accumulation of sodium carbonate minerals, again halite and
other highly soluble minerals precipitate. The ultimate brine is characterized by high concentrations of
RC0 3 and Si0 2 , apart from Na and Cl, and has a
high pR (9 to 11). Alkaline brines can dissolve considerable amounts of silica. If the lake water is diluted and the pR drops to lower values, water-rich
precursors of bedded chert or chert nodules are precipitated, as observed for example in Lake Magadi in
the East African rift zone (Eugster and Rardie 1978;
cf. Sect. 12.1.2).
- The residual brines, i.e., the brines after the precipitation of halite, may contain rare elements, such as
K, Li, U, P, B, F, Br, and J. This is in particular the
case, when the drainage area of the lake is rich in
volcanic rocks and primary minerals undergoing
first-cyc1e weathering. In several present-day salt
lakes, some of these elements are of economic interest.
In ancient lake deposits, the highly soluble constituents are
often missing, because they were dissolved by circulating
groundwater or dissipated by diffusion into neighboring
rocks. Furthermore, some of the primary salt minerals may
have been replaced by secondary, diagenetic minerals (e.g.
(1) The water volume of the lake with a given
amount of dissolved species is reduced. This occurs
when evaporation from the lake surface exceeds river
inflow and precipitation onto the lake surface (evaporative concentration, ephemerallake). As a result, the
lake shrinks and may even dry out and form an inland sabkha (see below). This process takes a relatively short time in which dissolved matter of further
inflowing water plays a minor role. The amount of
salt minerals which can be precipitated during one
cycle of evaporation therefore mainly depends on the
pre-existing volume and salt concentration of the
lake.
(2) The lake water volume remains more or less constant (perenniallake). The salt concentration slowly
increases due continuous river influx. Large drainage
areas with high chemical denudation rates (cf. Sect.
9.2) add more dissolved matter to the lake than small
ones in which weathering proceeds slowly.
Principally, mineral precipitation in a lake can commence when the concentrations of the dissolved species with respect to a specific mineral have reached
saturation (thermodynamic equilibrium). This process also depends on the concentrations of other dissolved species as well as on the temperature and pR
of the system. These relationships are special topics
in geochemistry.
The most important types of salt deposits and their
possible temporal and vertical successions in perennial and ephemeral lakes are summarized in Fig.
2.31. Since almost alllakes receive solid loads from
their drainage areas, reactions of so lids with lake water should be considered first. In fact, the incoming
clay minerals, degraded by weathering in the drainage areas, react with lake water and take up cations,
hydrogen carbonate and silica. This so-called
"re gradation" of clay minerals (or reverse weathering) consumes a significant amount of the incoming
dissolved matter.
It may lead to the formation of illite, montmorillonite, and
smectite at the expense of kaolinite. In addition, small
amounts of new clay minerals can form in the lake already
at very low salt concentrations. These processes have been
largely neglected in previous studies on lake evaporites.
The first mineral which is precipitated either chemically or biochemically is normal low-Mg calcite, regardless of the rock types present in the drainage areas. This is so because all river waters and shallow
groundwaters are rich in Ca and RC0 3 and mostly
show Ca>Mg and RC0 3 >Ca+Mg (cf. Sect. 9.2, Figs.
9.6 and 9.8). Increasing salt concentration, e1evated
temperature, and loss of CO 2 favor calcite precipitation.
The next steps in lake evolution are related to the
more specific chemistry of the inflowing water. Two
Chapter 2 Continental Sediments
or three different lines of deve10pment can be dis tinguished (Fig. 2.31):
- The inflowing water is of the common, earth alkalihydrogen carbonate type which is characteristic for
many rivers draining carbonate-bearing sedimentary
rocks and plutonic rocks in highlands. As a result of
calcite precipitation, Mg is re1atively enriched in the
solution. Prograding evaporation leads to the precipitation of further earth alkali carbonates (carbonate
lakes), but now Mg is partially used to transform calcite into other carbonate minerals, e.g. dolomite (see
below). Later, gypsum, halite, Mg salts and other
evaporites form. The ultimate brine is enriched in Na,
K, Mg, Cl and possibly S04 if sulfate reduction is
limited or absent. This brine is poor in RC03 and
Si0 2 and has a relatively low pR.
- A second, less common line of salt deposits starts
with waters comparatively rich in sulfate andlor chloride as well as calcium, but poor in RC03
(RCO)
evaporites in the drainage areas of the lakes (cf. Rio
Grande in North America, cf. Fig. 9.5). Calcite precipitation is followed by gypsum and NazS0 4 minerals inc1uding glauberite (sulfate lakes). The final
stage is again characterized by halite and other highly
soluble mineral:.;.
- The prerequisite for a third series of salts is lake
water rich in hydrogen carbonate and alkali ions.
Such waters often come from drainage areas dominated by volcanic and plutonic rocks inc1uding regions where Na is enriched by cation exchange in the
subsurface. After a sodalake stage with the accumulation of sodium carbonate minerals, again halite and
other highly soluble minerals precipitate. The ultimate brine is characterized by high concentrations of
RC0 3 and Si0 2 , apart from Na and Cl, and has a
high pR (9 to 11). Alkaline brines can dissolve considerable amounts of silica. If the lake water is diluted and the pR drops to lower values, water-rich
precursors of bedded chert or chert nodules are precipitated, as observed for example in Lake Magadi in
the East African rift zone (Eugster and Rardie 1978;
cf. Sect. 12.1.2).
- The residual brines, i.e., the brines after the precipitation of halite, may contain rare elements, such as
K, Li, U, P, B, F, Br, and J. This is in particular the
case, when the drainage area of the lake is rich in
volcanic rocks and primary minerals undergoing
first-cyc1e weathering. In several present-day salt
lakes, some of these elements are of economic interest.
In ancient lake deposits, the highly soluble constituents are
often missing, because they were dissolved by circulating
groundwater or dissipated by diffusion into neighboring
rocks. Furthermore, some of the primary salt minerals may
have been replaced by secondary, diagenetic minerals (e.g.